DESCRIPTION: Trafficking of RNA and protein between the nucleus and cytoplasm is essential to cell viability and it must be both efficient and well-controlled. The process of RNA export presents an opportunity to check th integrity of the products of transcription and processing, prior to their export to the cytoplasm. The applicant proposes that proofreading is a key feature in the export of RNAs. RNAs or RNPs that are present in excessive quantities, or that are defective as a consequence of mutation or incomplete o incorrect processing, are hypothesized to be retained and then degraded in the nucleus. To learn how various types of RNAs are exported from the nucleus and what features contributes to their proofreading, three specific aims will be pursued. These are: (1) to study the mechanisms by which tRNAs are exported an to determine what features of tRNAs are necessary and sufficient for their export; (2) to study the mechanisms by which the constitutive transport elemen (CTE) of MPMV is able to promote the release of incompletely spliced pre-mRNAs from spliceosomes and to relate export of that element to the process of norma mRNA export; and (3) to study the structure of a small RNA export element identified in a screen for exportable RNAs and determine what factors lead to its export or retention. For most of the experiments, Xenopus laevis oocytes will be used as a model system to identify and study transport factors, but some studies involve genetic selections in yeast. The focus will be on the structure and functions of cis-acting signals in RNA, their interactions with specific RNA export factors, and the mechanisms by which these factors promote export.